Hitherto, as a technology for realizing an oxide superconductor material, there has been a method in which a substrate is prepared, and an oxide superconductor is deposited on the substrate to obtain a superconducting oxide thin film.
For example, RE-based superconductors (RE: rare earth element) that exhibit superconducting phenomena at the temperature of liquid nitrogen (77K) or above, and in particular, yttrium-based superconductors represented by the composition formula YBa2Cu3O7-δ (represented by YBCO below), are often employed as oxide superconductors for forming a film. It is hoped that superconducting oxide thin films that employ such an RE-based superconductor will be applied to superconducting fault-current limiters, cables, superconducting magnetic energy storages (SMES), and the like, and RE-based superconductors and methods for their manufacture are attracting much attention.
Superconducting oxide thin films formed using a pure RE-based superconductor so as to have favorable crystal orientations, generally exhibit a characteristic of having a high critical current in the absence of a magnetic field. However, pure RE-based superconductors have a problem in that the critical current characteristic thereof rapidly decreases in high magnetic fields.
In order to enhance the critical current characteristic in a magnetic field, a pinning center is needed to prevent movement of quantized magnetic flux. Although normal conductor precipitates, stacking faults, dislocations, and the like are said to be effective pinning centers, it is not easy to produce a superconducting thin film while controlling these phenomena. In view of these circumstances, attempts has recently been made to introduce pinning centers artificially. It has been reported that artificially introducing BaZrO nanorods into YBCO as pinning centers enhances magnetic field characteristics (see, for example, Y. Yamada, K. Takahashi, H. Kobayashi, M. Konishi, T. Watanabe, A. Ibi, T. Muroga, S. Miyata, T. Kato, T. Hirayama, Y. Shiohara, “Epitaxial nanostructure and defects effective for pinning in Y(RE)Ba2Cu3O7-x coated conductors”, Appl. Phys. Lett., 2005, vol. 87, p. 132-502). However, in the case of nanorods, although the current characteristic at a specific angle under application of a magnetic field is high due to the presence of a one-dimensional normal conductive region, the current characteristic is lower at other angles. In order to isotropically increase the current characteristic under application of a magnetic field, it is preferable to introduce three-dimensional pinning centers formed by nanoparticles dispersed in three-dimensions (see, for example, Masashi Miura, Takeharu Kato, Masateru Yoshizumi, Yutaka Yamada, Teruo Izumi, Tsukasa Hirayama, and Yuh Shiohara: “Magnetic field angular dependence of critical current in Y1-xSmxBa2Cu3Oy coated conductors with nanoparticles derived from the TFA-MOD process”, TEION KOGAKU (J. Cryo. Soc. Jpn.) Vol. 44 No. 5 (2009)). Attempts have been in made to introduce pinning centers by doping YBCO with Al such that some of the Cu atoms in the CuO chains are replaced by Al atoms (see, for example, V. Antal, M. Kanuchova, M. Sefcikova, J. Kovac, P. Diko, M. Eisterer, N. Horhager, M. Zehetmayer, H. W. Weber, X. Chaud, “Flux pinning in Al doped TSMG YBCO bulk superconductors”, Supercond. Sci. Technol., 2009, vol. 22, 105001), and to substitute some of the Cu atom sites of YBCO with various metal elements (see, for example, Japanese Patent Application Laid-Open (JP-A) No. H07-330332).